FIG. 12 is a cross sectional view of a conventional surface acoustic wave filter. In FIG. 12, the conventional surface acoustic wave filter includes first terminal 101 at the input side and second terminal 102 at the output side, a plurality of resonators 103 electrically connected between first terminal 101 and second terminal 102, and piezoelectric substrate 105 provided at the top surface of the above-described resonators 103. A signal of 1800 MHz˜2300 MHz is inputted to first terminal 101 at the input side. Piezoelectric substrate 105 is mounted flip chip on base substrate 104 to form a package, which is covered with sealing 106. A resin material is used for sealing 106 in order to secure the adhesion of sealing 106 with base substrate 104. Japanese Patent Unexamined Publication No. H9-74329 and Japanese Patent Unexamined Publication No. H10-22763 are the examples of prior art documents related to the present patent application.
The filtering characteristic of the conventional surface acoustic wave filter, however, remained unsatisfactory. FIG. 13 shows transit characteristic of a conventional surface acoustic wave filter. As FIG. 13 shows in zone A, it demonstrates poor attenuation characteristic at 2140 MHz and the neighborhood, which deteriorated the filtering characteristic.